CO2chemosensitivity inHelix aspersa: three potassium currents mediate pH-sensitive neuronal spike timing

Abstract

Elevated levels of carbon dioxide increase lung ventilation in Helix aspersa. The hypercapnic response originates from a discrete respiratory chemosensory region in the dorsal subesophageal ganglia that contains CO2-sensitive neurons. We tested the hypothesis that pH-dependent inhibition of potassium channels in neurons in this region mediated the chemosensory response to CO2. Cells isolated from the dorsal subesophageal ganglia retained CO2chemosensitivity and exhibited membrane depolarization and/or an increase in input resistance during an acid challenge. Isolated somata expressed two voltage-dependent potassium channels, an A-type and a delayed-rectifier-type channel ( IKAand IKDR). Both conductances were inhibited during hypercapnia. The pattern of voltage dependence indicated that IKAwas affected by extracellular or intracellular pH, but the activity of IKDRwas modulated by extracellular pH only. Application of inhibitors of either channel mimicked many of the effects of acidification in isolated cells and neurons in situ. We also detected evidence of a pH-sensitive calcium-activated potassium channel ( IKCa) in neurons in situ. The results of these studies support the hypothesis that IKAinitiates the chemosensory response, and IKDRand IKCaprolong the period of activation of CO2-sensitive neurons. Thus multiple potassium channels are inhibited by acidosis, and the combined effect of pH-dependent inhibition of these channels enhances neuronal excitability and mediates CO2chemosensory responses in H. aspersa. We did not find a single “chemosensory channel,” and the chemosensitive channels that we did find were not unique in any way that we could detect. The protein “machinery” of CO2chemosensitivity is probably widespread among neurons, and the selection process whereby a neuron acts or does not act as a respiratory CO2chemosensor probably depends on the resting membrane potential and synaptic connectivity.